A variety of coupling methods have been used to realize tightly coupled structures for microstrip and stripline applications. The conductors may be placed in close proximity to one another in order to achieve coupling. However, when hard substrates are employed, tight coupling can usually be obtained only with interdigitated structures. Even where this technique is used, coupling values tighter than -3 dB are still difficult to obtain in planar media which include microstrip.
The interdigitated structure currently used to realize tight coupling requires complex processes in its fabrication. Further, the use of an interdigitated structure gives rise to high losses due to the required conductor narrowing mandated by interdigitated techniques. Standard coupling techniques utilizing proximate placement of the conductors in order to achieve coupling require accurate control of the gap between the two coupled conductors. Further, there are fabrication limitations which prevent the conductors from being constructed too close to one another. Additionally, where the conductors are relatively close to one another, there exists a possibility of voltage breakdown therebetween.
Therefore, a need has arisen for a coupler circuit which may be utilized in order to tightly couple microstrip conductors while eliminating the stringent requirements for fabrication of the prior interdigitated and proximate conductor configurations.